Dry skin conductance electrode

ABSTRACT

The present invention relates to a dry skin conductance electrode ( 12, 13 ) for contacting the skin ( 10 ) of a user. In order to provide a dry skin conductance electrode for long-term measurements which does not cause problems to the user while still providing a good signal level, the electrode ( 12, 13 ) comprises a material made of a noble metal doped with at least one dopant selected from the group consisting of hydrogen, lithium, sodium, potassium, rubidium, caesium and beryllium. The present invention also relates to a skin conductance sensor ( 20 ), a wristband ( 30 ) and an emotional event detection system.

FIELD OF THE INVENTION

The present invention relates to a dry skin conductance electrode forcontacting the skin of a user and to a skin conductance sensorcomprising at least two dry electrodes, wherein the sensor is adapted tosense a user's skin conductance between the at least two dry electrodes.The present invention also relates to a wristband comprising such a skinconductance sensor and an emotional event detection system comprisingsuch a skin conductance sensor.

BACKGROUND OF THE INVENTION

It is known that skin conductance of a user is related with the level ofarousal of a user. Everything that emotionally touches the useractivates the sweat glands in the skin leading to a better conductorpath through the skin. For example, in a known lie detector orpolygraph, a skin conductance sensor connected to the palm of the handor to the fingers is used.

Commonly, gel electrodes are used for skin conductance sensors. Thesegel electrodes offer a high signal level. However, prolonged wearing ofgel electrodes causes undesirable side effects, such as a white swellingof the skin caused by hydration.

When the period of measurement is long, the skin conductance sensorneeds to be comfortable for the user. In the article “A Wearable Sensorfor Unobtrusive, Long-Term Assessment of Electrodermal Activity”,Ming-Zher Poh, Nicholas C. Swenson, and Rosalind W. Picard, IEEETransactions on Biomedical Engineering, Vol. 57, No. 5 (2010) 1243-1252,a wrist-worn integrated sensor is disclosed. This sensor has Ag/AgClelectrodes, and no conductive gel applied to the electrodes. However,due to the Ag/AgCl material, prolonged wearing of this device causesbrown skin coloration due to the injection of silver ions into the skin,which is an undesirable side effect.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide, for long-termmeasurements, a dry skin conductance electrode, as well as a skinconductance sensor, wristband and emotional event detection systemcomprising such dry electrodes, which does not cause problems to theuser, such as skin irritation or skin coloration, while still providinga good signal level.

In a first aspect of the present invention, a dry skin conductanceelectrode for contacting the skin of a user is presented, the electrodecomprising a material made of a noble metal doped with at least onedopant selected from the group consisting of hydrogen, lithium, sodium,potassium, rubidium, caesium and beryllium.

In a second aspect of the present invention, a skin conductance sensorcomprising at least two dry electrodes is presented, that is adapted tosense a user's skin conductance between the at least two dry electrodes,wherein at least one of the electrodes is the dry skin conductanceelectrode as described above.

In a further aspect of the present invention a wristband is presentedthat comprises such a skin conductance sensor.

In a still further aspect of the present invention an emotional eventdetection system for detecting an emotional event of a user ispresented, that comprises such a skin conductance sensor, a transmissionlink for transmitting data indicative of the sensed skin conductance,and a processing unit adapted to process the transmitted data and detectan emotional event in the user based on the transmitted data.

The present invention is based on the idea to provide a skin conductancesensor for long-term measurements (for example several hours or days)comprising dry electrodes with a good non-polarizable electronicskin-electrode interface. A dry skin conductance electrode is anelectrode which does not require the use of a conductance gel for skinconductance measurements, thus also called gel-free skin conductanceelectrode. The dry, gel-free skin conductance electrode makes directcontact with the skin of the user, thus forming a skin-electrodeinterface. The skin-electrode interface is an interface between a mediumwhere the current carriers are predominantly electronic (electrode), anda medium where the current carriers are predominantly ionic (skin).Usually, such an interface suffers from a poor charge transfer, leadingto the formation of a space charge. A non-polarizable skin-electrodeinterface is provided when there is charge transfer at the interface. Aperfectly non-polarizable interface would exhibit no impedance to thecharge transfer, which is however not possible in practice. At anon-polarizable skin-electrode interface, due to an electrochemicalreaction, ions are injected into the skin from the positive electrode.Thus, electrons are left in the positive electrode, causing a currentflow to take place. At the negative electrode, ions are absorbed fromthe skin, in particular protons, sodium ions or potassium ions, as thefluid secreted by sweat glands contains mainly hydrogen, sodium andpotassium. The ions are incorporated into the metal matrix of thenegative electrode material as atoms, after the acceptance of anelectron.

Since the dry skin conductance electrode comprises a noble metal dopedwith at least one dopant selected from the group consisting of hydrogen,lithium, sodium, potassium, rubidium, caesium and beryllium, the chargetransfer process (ionic exchange between the skin and the electrodematerial) and thus the interface is improved, leading to a goodnon-polarizable interface. No gel is needed and no skin problems arecaused, such as skin coloration due to the injection of silver ions tothe skin.

A dopant is generally a trace impurity element that is inserted into abase material in very low concentrations, for example in order to altera specific property of the base material. In the claimed material, anoble metal is used as base material, since these metals are most likelynot to take part in the electrochemical reaction with the skin. Ingeneral, noble metals are the group of ruthenium (Ru), rhodium (Rh),palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt),and gold (Au). The noble metal of the claimed material should not causedermatological problems to the user. In general, the claimed materialshould be non-toxic, or at a toxic level which is below a value that isharming to the user. For example, even though beryllium (Be) is toxic,it can be used as the dopant, if the concentration of the dopant isbelow a value that is harming to the user.

Preferred embodiments of the invention are defined in the dependentclaims. It shall be understood that the claimed skin conductance sensor,wristband or emotional detection system has similar and/or identicalpreferred embodiments as the claimed skin conductance electrode and asdefined in the dependent claims.

According to a first embodiment, the noble metal is at least one elementselected from the group of gold, palladium and platinum. These noblemetals are especially suitable in combination with the dopants mentionedabove.

Further, in a second embodiment, the dopant is lithium, sodium orpotassium. These dopants are especially suitable when used in noblemetals. In particular, the atoms of these dopants have a relatively highmobility due to their relatively low atomic radius.

Any combination of the above mentioned elements of the first embodimentand the elements of the second embodiment is possible. In oneembodiment, the dopant is an element from the first group of theperiodic table or monovalent. Further, in an embodiment, the dopant isan alkali metal. Alkali metals are generally the group of lithium (Li),sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium(Fr).

In a preferred embodiment, the material is made of gold, palladium orplatinum and the dopant is lithium, therefore: gold doped with lithium,palladium doped with lithium or platinum doped with lithium. Since thelithium atoms have a low atomic radius and thus a high mobility,diffusion processes are minimized.

In another preferred embodiment, the material is made of gold and thedopant is lithium, sodium or potassium.

In a further embodiment, the difference in ionization potential betweenthe noble metal and the dopant is at or above a level at which the noblemetal is prevented from taking part in the electrochemical reaction withthe skin. Thus, it is prevented that ions of the noble metals are alsoinjected into the skin due to electrochemical reaction.

According to a further embodiment, the concentration of the dopant isbetween 0.1 and 5%, in particular between 0.5 and 3%, in particularbetween 0.7 and 1.3%, in particular about 1%±0.2%. It is achieved thatthe concentration of the dopant is sufficiently high to preventdepletion and sufficiently low not to change the main properties of thenoble metal.

In a still further embodiment, the material is located at an outersurface of the electrode for interfacing the skin. This enables that theclaimed material is in direct contact with the skin.

In another embodiment, the electrode comprises an outer layer which isformed of the material. Thus, only a thin outer layer of the claimedmaterial is needed which reduces costs, as the remaining part of theelectrode can be made of a cheaper material.

In a variant of the embodiment above, the electrode further comprises aninner layer beneath the outer layer. This can provide more stability tothe electrode and can reduce manufacturing costs.

In particular, in this variant, the inner layer is formed of nickeland/or brass. This reduces manufacturing costs, as these materials aregenerally cheaper than noble metals.

In an embodiment of the skin conductance sensor, the skin conductancesensor comprises a voltage generator for applying a voltage between theat least two dry electrodes, a measuring means for measuring a currentbetween the at least two dry electrodes, and a calculating unit forcalculating skin conductance based on the measured current. Thisprovides for a skin conductance sensor that is easy to implement.Preferably, the applied voltage is a constant voltage.

In an embodiment of the skin conductance sensor, the two dry electrodescomprise the same material, in particular the claimed material. In analternative embodiment of the skin conductance sensor, the two dryelectrodes comprise different materials. In one embodiment, the positiveelectrode comprises the claimed material. Alternatively or cumulatively,the negative electrode comprises the claimed material.

In an embodiment of the wristband or the skin conductance sensor, the atleast two dry electrodes are arranged for contacting the volar side ofthe wrist of the user. Hence, good measurement can be obtained, sincethe volar side of the wrist is a region in which the skin conductance isat the same level. Also there is generally no hair, which couldinfluence the measurement, in this region.

In an embodiment of the emotional event detection system or the skinconductance sensor, the transmission link is a wireless link between theskin conductance sensor and the processing unit. This enables to providefor a comfortable mobile system.

In an embodiment of the emotional event detection system, the processingunit is adapted to detect a peak having a particular rising slope and/ora particular down slope in the transmitted skin conductance data. Skinconductance is related with the level of arousal of the user. Hence, aneasy way of determining emotional events from skin conductance data isprovided.

In a still further embodiment of the emotional detection system, thesystem comprise at least one further sensor, such as a heart ratesensor, for example for measuring heart rate variations, a breathingsensor, a blood sensor, a body temperature sensor, a voice sensor, acamera for capturing the face of the user, or the like. This enables tocombine measurements from the skin conductance sensor with measurementsof other sensors. Thus, accuracy of the emotional event detection isimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter. Inthe following drawings

FIG. 1 shows an interface between skin and two dry electrodes accordingto an embodiment of the present invention;

FIG. 2 shows a cross section of a skin conductance electrode accordingto an embodiment of the present invention;

FIG. 3 shows a first skin conductance trace, sensed by a skinconductance sensor according to an embodiment of the present invention,and a second skin conductance trace for comparison;

FIG. 3 a shows two skin conductance traces for comparison;

FIG. 3 b shows a skin conductance trace, sensed by a skin conductancesensor according to an embodiment of the present invention;

FIG. 4 shows a schematic block diagram of a skin conductance sensoraccording to an embodiment of the present invention;

FIG. 5 shows an illustration of a wristband according to an embodimentof the present invention;

FIG. 6 shows an illustration of an emotional event detection systemaccording to an embodiment of the present invention;

FIG. 7 shows a skin conductance trace, sensed by a skin conductancesensor according to an embodiment of the present invention, fordetermining emotional events.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an interface between skin 10 of a user and two dryelectrodes 12, 13 according to an embodiment of the present invention.For example, when the user wears a skin conductance sensor 20 comprisingthe two dry electrodes 12, 13, the two dry electrodes 12, 13 are placedon the skin of the user. A voltage 11 is applied between the two dryelectrodes 12, 13 such that a positive electrode 12 and a negativeelectrode 13 is provided. The positive electrode 12 has an outer surface15 which interfaces with the skin 10 and the negative electrode 13 hasan outer surface 19 which interfaces with the skin 10. Theskin-electrode interface is an interface between a medium where thecurrent carriers are predominantly electronic (electrodes 12, 13 in FIG.1), and a medium where the current carriers are predominantly ionic(skin 10 in FIG. 1). Usually such an interface suffers from a poorcharge transfer, leading to the formation of a space charge.

However, as can be seen in FIG. 1, a non-polarizable skin-electrodeinterface is provided as there is charge transfer at the interface. Thedry skin conductance electrodes 12, 13 each comprise a material made ofa noble metal, marked with M, doped with at least one dopant, markedwith D1, selected from the group consisting of hydrogen, lithium,sodium, potassium, rubidium, caesium and beryllium. Due to anelectrochemical reaction, ions of the dopant, marked with D1+, areinjected into the skin 10 from the material of the positive electrode12. Thus, electrons are left in the positive electrode, causing acurrent flow to take place. At the negative electrode 13, ions, markedwith D2+, are absorbed from the skin. These ions D2+ are in particularhydrogen, sodium or potassium, as the fluid secreted by sweat glandscontains mainly hydrogen, sodium and potassium. The ions D2+ areincorporated into the metal matrix of the material of the negativeelectrode 13 as atoms D2, after the acceptance of an electron.Therefore, the charge transfer process and thus the interface isimproved, leading to a good non-polarizable interface. The ionicexchange between the skin 10 and the material of the electrode 12, 13 isfacilitated.

In the embodiment of FIG. 1, the two dry electrodes 12, 13 of the skinconductance sensor comprise different materials at their outer surfaces15, 19. Positive electrode 12 comprises a material made of a noble metalM1 doped with a dopant D1 and negative electrode 13 comprises a materialmade of a noble metal M2 doped with a dopant D2. In an alternativeembodiment, the two dry electrodes 12, 13 can comprise the samematerial. For example, when the material is made of palladium doped withlithium (Pt—Li), this material can be used for both electrodes, thepositive electrode 12 and the negative electrode 13, as it is optimalfor both electrodes.

In a first embodiment, the noble metal is gold, palladium or platinum,or any combination, thus any alloy. In a second embodiment, the dopantis lithium, sodium or potassium. In another embodiment the dopant is anelement from the first group of the periodic table or monovalent, inparticular an alkali metal, thus from the group of lithium (Li), sodium(Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Inanother embodiment, also beryllium (Be), even though toxic, can be usedas the dopant, if the concentration of the dopant is below a value thatis harming to the user.

In a preferred embodiment, the material is made of gold doped withlithium (Au—Li), palladium doped with lithium (Pd—Li) or platinum dopedwith lithium (Pt—Li). In another preferred embodiment, the material ismade of gold and the dopant is lithium, sodium or potassium, therefore:gold doped with lithium (Au—Li), gold doped with sodium (Au—Na) or golddoped with potassium (Au—K). In a most preferred embodiment, thematerial is made of gold doped with lithium (Au—Li, for example with adopant concentration between 0.1 and 5%, in particular between 0.5 and3%, in particular between 0.7 and 1.3%, in particular 1%±0.2%. In thiscase for example, the difference in ionization potential between thenoble metal and the dopant is at or above a level at which the noblemetal is prevented from taking part in the electrochemical reaction withthe skin 10. Thus, it is prevented that ions of the noble metal are alsoinjected into the skin 10 due to electrochemical reaction.

FIG. 2 shows a cross-section of a skin conductance electrode 12, 13according to an embodiment. The electrode 12, 13 has an outer surface15, 19 for interfacing with the skin 10. The material described above islocated at the outer surface 15, 19. In FIG. 2, the electrode 12, 13comprises an outer layer 14 which is formed of the material. The outerlayer 14 comprises the outer surface 15 for interfacing with the skin10. The electrode 12, 13 further comprises a first inner layer 16located beneath the outer layer 14. The electrode 12, 13 furthercomprises a second inner layer, or base layer, 18 located beneath thefirst inner layer 16.

In the most preferred embodiment, the second inner layer, or base layer,18, is formed of brass, the first inner layer 16 is a formed of nickel,and the outer layer 14 is formed of a material made of gold doped withmonovalent lithium ions (Au—Li)

In an exemplary manufacturing method, the material of the second innerlayer 18, such as brass in the most preferred embodiment, for example inthe form of a plate, is polished and electrochemically plated with thefirst inner layer 16, such as nickel in the most preferred embodiment.Then, the outer layer 14, such as gold doped with lithium in the mostpreferred embodiment, is applied by sputtering. Optionally, beforesputtering the outer layer 14, the material can be melted, such as in aclosed quartz vessel, then cooled, afterwards flattened and the sputtertargets, for example of round form, can be cut out. Also optionally,before sputtering the outer layer 14, the surface of the first innerlayer 16 can be cleaned using reactive ion etching in order to improvethe bonding between the first inner layer 16 and the outer layer 14. Analternative to applying the outer layer 14 by sputtering is theco-deposition in vacuum by evaporation and/or e-beam deposition. Forexample, gold can be e-beam deposited and lithium can be deposited invacuum by evaporation from a heated crucible, due to the low meltingpoint of lithium. Optionally, the thickness of the layer can bemonitored so that the deposition speed can be controlled. A goodstability of the outer layer 14 can thus be realized throughout thelayer. The thickness of the outer layer 14 can for example be in theorder of microns, in particular less than 1 micron.

FIG. 3 shows skin conductance traces. The measured skin conductancevalues over time form a skin conductance trace. The x-axis is the timeaxis, for example measured in minutes (min), and the y-axis is the skinconductance axis, for example measured in microSiemens (μS). Skinconductance, or also called galvanic skin response (GSR), is a measureof the electrical conductance of the skin, which varies with itsmoisture level, thus the sweat gland activity.

In particular, FIG. 3 shows a first skin conductance trace, sensed by askin conductance sensor according to an embodiment, namely the mostpreferred embodiment described above, using a material made of golddoped with lithium at the outer surface of the electrode. This firstskin conductance trace is illustrated by a plain solid line. Forcomparison, FIG. 3 also shows a second skin conductance trace,illustrated by the circled solid line, in which a conventional electrodehaving nickel at the outer surface has been used. The improved signallevel of the first skin conductance trace (Au—Li) compared to the secondskin conductance trace (Ni) can be clearly seen in FIG. 3. If an undopedgold material (Au) at the outer surface of the electrode material wouldbe used, the signal level of the corresponding skin conductance trace(not shown in FIG. 3) would be even lower than with the nickel (Ni)electrode. For mere comparison purposes, this is illustrated in FIG. 3 ashowing a skin conductance trace sensed by a skin conductance sensorhaving a conventional nickel (Ni) electrode and a skin conductance tracesensed by a skin conductance sensor having a gold (Au) electrode.

By using an electrode comprising a material made of gold doped withpotassium (Au—K) or gold doped with sodium (Au—Na), similar results aswith a material made of gold doped with lithium (Au—Li) can be obtained.However, with potassium and sodium, there is a large period of signalincrease, compared to the use of lithium, after first use due todiffusion processes. More time is needed for the skin conductance toreach a stable level, compared to the Au—Li example shown in FIG. 3.This is exemplary illustrated in FIG. 3 b, showing a skin conductancetrace, sensed by a skin conductance sensor comprising an electrodecomprising gold doped with sodium (Au—Na). The slow increase of thesignal can be seen in FIG. 3 b. This can be explained with thedifference in radius between sodium, potassium and lithium: potassiumradius: 138 μm, sodium radius: 102 μm, lithium radius: 76 μm. Thesmaller the ion is, the easier it is to penetrate the skin due to itsincreased mobility.

Similar results as shown for gold doped with lithium, potassium orsodium can also be obtained by doping platinum with lithium, potassiumor sodium.

FIG. 4 shows a schematic block diagram of a skin conductance sensor 20according to an embodiment. The skin conductance sensor 20 comprises twodry electrodes 12, 13. The skin conductance sensor 20 further comprisesa voltage generator 22 for applying a voltage 11, in particular aconstant voltage, between the two electrodes 12, 13. Normally, a smallvoltage is applied, for example less than 1.2 V. Such a small voltageinduces a small current, for example in the order of 1 μA, through theskin. The skin conductance sensor 20 further comprises a measuring unit24 for measuring a current or voltage drop between the two dryelectrodes 12, 13. An A/D converter 25 of the skin conductance sensor 20digitizes the measured current or voltage drop. The skin conductancesensor further comprises a calculating unit 26, such as a processor, forcalculating a skin conductance based on the measured current or voltagedrop. It should be understood, that also the skin resistance can becalculated which is the inverse of the skin conductance.

The measured skin conductance values, or the skin conductance trace, canfor example be transmitted by a transmitter 28 over a wirelesstransmission link. Additionally or alternatively, these measured skinconductance values can be stored in a memory unit 29.

The skin conductance sensor 20 comprises a casing 27. All or only someof the components described above can be integrated in the casing 27.However, some components may also be separate parts. In particular, theelectrodes 12, 13 can be separate parts.

FIG. 5 shows an illustration of a wristband according to an embodiment.The wristband 30 comprises a skin conductance sensor 20 as describedabove, in particular as illustrated in FIG. 4. The wristband 30comprises a wristband material part 31, for example made of a textile orplastic, which loops around the wrist of the user. Even though calledwristband, the wristband 30 could also be worn around the ankle or othersuitable body part. The two dry electrodes 12, 13 are integrated intothe wristband 30, in particular integrated into the wristband materialpart 31. The two dry electrodes 12, 13 are arranged such that theycontact the volar side of the wrist when the wristband 30 is worn by theuser. The electrodes 12, 13 have a round form in FIG. 5, however anyother suitable electrode form may be used. In particular, the electrodes12, 13 in FIG. 5 are in the form of clothing buttons. A casing 27comprises a voltage generator 22, a measuring unit 24, and a calculatingunit 26. The two electrodes 12, 13 are separate parts and are connectedto the casing 27 by means of wires located within the wristband.Alternatively, the two electrodes 12, 13 can also be integrated into thecasing 27. In FIG. 5, a transmitter 28 for wirelessly transmitting themeasured skin conductance values is integrated into the casing 27.

FIG. 6 shows an illustration of an emotional event detection systemaccording to an embodiment. The emotional event detection system detectsan emotional event of a user. The system comprises a skin conductancesensor 20, which is integrated into the wristband 30 shown in FIG. 6.The wristband 30 shown in FIG. 6 can for example be the wristband 30 ofthe embodiment in FIG. 5. The system further comprises a processing unit34 adapted to process the transmitted data and detect an emotional eventin the user based on the transmitted data. The processing unit 34 can bea separate part. The system also comprises a transmission link 32,between the skin conductance sensor 20 and the processing unit 34, fortransmitting data indicative of the sensed skin conductance, for examplethe measured skin conductance values. The transmitter in the wristband30, such as transmitter 28 in FIG. 5 or FIG. 4, transmits dataindicative of the sensed skin conductance to a receiver of theprocessing unit 34 over the wireless transmission link 32. Thetransmission link 32 illustrated in FIG. 6 is a wireless link. However,also other transmission links are possible, such as transmission bydownloading data from a memory or using a cable. Even though notillustrated in FIG. 6, the system can comprise at least one furthersensor, such as a heart rate sensor, for example for measuring heartrate variations, a breathing sensor, a blood sensor, a body temperaturesensor, a voice sensor, a camera for capturing the face of the user, orthe like. Each of these further sensors can be adapted to transmit thesensed data to the processing unit 34. The processing unit 34 can thencombine the measurements from the skin conductance sensor withmeasurements of other sensors, in order to improve the accuracy of theemotional event detection.

FIG. 7 shows a skin conductance trace sensed by a skin conductancesensor according to an embodiment, in particular measured with the skinconductance sensor or wristband as previously described. The skinconductance trace shows several hours of measurement. The processingunit 34 shown in FIG. 6 is adapted to detect a particular rising slopeand/or a particular down slope in the transmitted skin conductance data,in particular detecting peaks with a steeper rising slope and moregentle down slope. In this way, an emotional event can be detected. Skinconductance is related with the level of arousal of the user. Everythingthat emotionally touches the user activates the sweat glands in the skinleading to a better conductor path through the skin. Hence, an easy wayof determining emotional events from skin conductance data is provided.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limitingthe scope.

1. Skin conductance sensor comprising at least two dry electrodes,wherein the sensor (20) is adapted to sense a user's skin conductancebetween the at least two dry electrodes, wherein at least one of theelectrodes is a dry skin conductance electrode for contacting the skinof a user, the electrode comprising a material made of a noble metaldoped with at least one dopant selected from the group consisting ofhydrogen, lithium, sodium, potassium, rubidium, caesium and berylliumwherein the material is located at an outer surface of the electrode forinterfacing with the skin.
 2. Skin conductance sensor of claim 1,wherein the noble metal is an element selected from the group consistingof gold, palladium and platinum.
 3. Skin conductance sensor of claim 1,wherein the dopant is lithium, sodium or potassium.
 4. Skin conductancesensor of claim 1, wherein the noble metal is gold, palladium orplatinum, and the dopant is lithium.
 5. Skin conductance sensor of claim1, wherein the difference in ionization potential between the noblemetal and the dopant is at or above a level at which the noble metal isprevented from taking part in the electrochemical reaction with theskin.
 6. Skin conductance sensor of claim 1, wherein the concentrationof the dopant is between 0.1 and 5%.
 7. (canceled)
 8. Skin conductancesensor of claim 1, wherein the electrode comprises an outer layer whichis formed of the material.
 9. Skin conductance sensor of claim 8,wherein the electrode further comprises at least one inner layer beneaththe outer layer.
 10. (canceled)
 11. Skin conductance sensor of claim 1further comprising: a voltage generator for applying a voltage betweenthe at least two electrodes, a measuring unit for measuring a currentbetween the at least two electrodes and a calculating unit forcalculating skin conductance based on the measured current. 12.Wristband comprising the skin conductance sensor of claim
 1. 13.Wristband of claim 12 wherein the at least two electrodes are arrangedfor contacting the volar side of the wrist of the user.
 14. Emotionalevent detection system for detecting an emotional event of a user, thesystem comprising: the skin conductance sensor claim 1, a transmissionlink for transmitting data indicative of the sensed skin conductance,and a processing unit adapted to process the transmitted data and detectan emotional event of the user based on the transmitted data. 15.Emotional event detection system of claim 14, wherein the transmissionlink is a wireless link between the skin conductance sensor and theprocessing unit.